Find an Equation for the Graph $y = 3 \cos \left( \frac { 1 } { 2 } x \right)$

Multiple Choice

Find an equation for the graph.
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A) $y = 3 \cos \left( \frac { 1 } { 2 } x \right)$
B) $y = 3 \cos ( 2 x )$
C) $y = 2 \cos ( 3 x )$
D) $y = 5 \cos \left( \frac { 1 } { 2 } x \right)$